Do not use distribute in requirements gate venv

Remove the --distribute flag so that the virtualenv
is created with vanilla setuptools to avoid conflicts
between packages that try to update setuptools or
distribute.

Change-Id: I63456887027f707a6b42c86ea44e587df0f503de
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Reviewed-on: https://review.openstack.org/35905
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins